1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and particularly to an apparatus having a NOx removing device and a particulate filter (hereinafter referred to as “DPF”) for trapping particulates (particulate matter) in exhaust gases of the internal combustion engine.
2. Description of the Related Art
The NOx removing device, which traps NOx in an oxidizing state where a concentration of oxygen in exhaust gases is relatively high, and reduces NOx in a reducing state where a concentration reducing components in exhaust gases is comparatively high, is well known. The NOx removing device traps not only NOx but also sulfur oxide generated by oxidization of sulfur contained in fuel, resulting in reduction of a trapping capacity. Therefore, when an amount of trapped sulfur oxide becomes large, it is necessary to perform a process for removing the trapped sulfur oxide.
On the other hand, the DPF is widely used for trapping particulates (particulate matter) which mainly consist of soot in exhaust gases. Since there is a limit in the amount of particulates that can be trapped in the DPF, it is necessary to timely perform a DPF regeneration process for burning the particulates trapped in the DPF.
Japanese Patent Laid-open No. 2005-188387 discloses a method for alternately performing the above-described sulfur oxide removal process and DPF regeneration process. Specifically, the temperature of the DPF is maintained at a temperature which is equal to or greater than a temperature sufficient for burning the particulates and the exhaust gases are controlled to be in the oxidizing state, to perform the DPF regeneration. Subsequently, the temperature of the NOx removing device is maintained at a temperature which is sufficient for removing sulfur oxide and the exhaust gases are controlled to be in the reducing state, to perform the sulfur oxide removal.
In the above-described conventional method, a repetition time period of the sulfur oxide removal process and the DPF regeneration process is set to a predetermined time period or a time period set according to an amount of particulates accumulated in the DPF (a residual amount of particulates in the DPF). However, it is difficult to control the exhaust gases to be alternately in the oxidizing state and in the reducing state while controlling the temperatures of the NOx removing device and the DPF to be desired values. Therefore, there is a possibility that the temperature of the NOx removing device or other catalysts for purifying exhaust gases may become excessively high, thereby accelerating deterioration of the NOx removing device or other exhaust gas purifying catalysts.